Return an element of the container uniformly at random using
the generator.
| sample-element < > < cl-variates:basic-random-number-generator > |
| sample-element < sequence > < cl-variates:basic-random-number-generator > |
| sample-element < t > < cl-variates:random-number-generation-mixin > |
| sample-element (eql nil)< cl-variates:basic-random-number-generator > |